Exemplary embodiments of the present invention relate to a spark plug or igniter for an internal combustion engine, and more particularly to a spark plug or igniter that initiates combustion, facilitates combustion control and burns off soot deposits in a diesel engine.
Soot is a common byproduct of the incomplete combustion of fuel in internal combustion engines namely, diesel engines. In particular, conventional fuels are comprised of hydrocarbons, which after undergoing complete combustion, produce byproducts of only carbon dioxide and water. However, complete combustion does not typically occur in internal combustion engines since no known engine is entirely efficient. In addition, complete combustion can require a lean fuel-air mixture whereas typical engine conditions require richer fuel-air mixtures to produce a desired performance.
Further emission regulations are mandating the use of new engine combustion cycles such as homogeneous charge compression ignition (HCCI) and exhaust treatment systems for diesel engines. These new combustion cycles will require new methods for combustion sensing and control. There may also be certain engine load conditions where more conventional combustion cycles still work best. For these conditions, spark assist is one means of controlling the combustion process. This unique combination of needs for in-cylinder combustion sensing and combustion initiation can be supported with a spark plug designed to work well in the higher pressure diesel engine cylinder environment as an igniter and also as an ion sensor for combustion feedback and control. For exhaust treatment, better methods are needed to actively regenerate the particulate filters. One method for active regeneration is to provide a self contained burner system to add heat energy to the exhaust to initiate the regeneration cycle. This burner system requires a reliable igniter that can survive in the corrosive and turbulent diesel exhaust environment.
In addition, soot typically accumulates at a higher rate in diesel engines than in gasoline engines due to the different ways that fuel is injected and ignited. In particular, in gasoline engines, fuel is injected during the intake stroke and thoroughly mixed with air before ignition by a spark. Conversely, in diesel engines, fuel is injected during the compression stroke and ignited spontaneously from the pressure. In that respect, combustion occurs at the boundary of unmixed fuel, where localized pockets of rich fuel-air mixtures are ignited, thus producing soot.
Soot deposits can accumulate on insulator tips of conventional spark plugs used as an igniter in a burner system. The exposed surface of the insulator tip is typically located at or near the boundary of unmixed fuel. Moreover, the exposed surface of the insulator tip is not typically located in or about the spark gap between the side electrode and the center electrode. In particular, the typical spark plug includes a center electrode extending past an insulator tip and a side electrode extending past the center electrode. For these reasons, soot may accumulate on the insulator tip and may not be burned off.
Accordingly, it is desirable to provide a spark plug or igniter design that is more robust than conventional spark plug designs to high cylinder pressures and to the corrosive effects of the combustion chamber or exhaust.